In a remotely illuminated phosphor package, light from one or more ultraviolet or blue pump LEDs is imaged by a hemispherical lens onto a phosphor-containing layer that is approximately adjacent and coplanar to the LEDs. Excellent color uniformity for a single LED, and within a lot of LEDs can be achieved by using precise and accurate optical coatings over the surface of the hemispherical surface that reflect a large percentage of blue light and transmit a high percentage of red and green light.
Two methods of making optical coatings have proved to provide the necessary coating precision and accuracy required with remotely illuminated phosphor packages; low pressure chemical vapor deposition (LPCVD) and atomic layer deposition (ALD). LPCVD uses reactive gases at about 1 Torr of pressure to deposit typically inorganic oxide coatings on a refractory surface. LPCVD provides high deposition speeds and excellent coating uniformity over a single optical element. LPCVD deposition is determined by reaction kinetics at solid surfaces, so gas velocity and history affect coating speed. The effect of the flow history has a significant effect on coating precision. ALD uses saturated physisorption or chemisorption, or a combination of both, to limit coating thickness in any one coating step. Very accurate and precise coatings can be produced using ALD, assuming that all surfaces have enough exposure to the reactant to develop a saturated adsorbed layer of reactant or reactant-modified surface, and that the purge time is sufficient to remove excess reactant and reaction products.
Both coating processes can be relatively time consuming, with LPCVD taking 1 or more hours to develop a micron thick coating, and ALD taking 10 hrs or more for the same coating thickness. Therefore, in order to produce low cost coatings, a significant number of optical elements must be coated at the same time. However, loading a number of optical elements into a reactor can create non-uniform gas flows. For LPCVD, this can cause some optical elements to have too thick a coating, and some elements too thin a coating. While ALD is less affected by the non-uniform gas flow, the coating time can be substantially lengthened due to the fact that the optical element surface receiving the least amount of gas flow limits the coating speed.
The way that the optical elements fill the reactor can also have a significant effect on coating cost and quality. For example, a regular array with spaces can give high quality coatings, but at the cost of lower packing density. Reducing packing density reduces the potential throughput of the system, and assuming that the substrates or optical elements take up volume, can increase mixing within the reactor. Mixing can increase reactant and purge cycle times in ALD systems. Reduced packing density also reduces throughput of both ALD and LPCVD systems.
Another approach to filling reactors is to create a randomly packed volume of elements. This approach is used for coating powders. One advantage of random packing is that the reactor is inexpensive to build and can be quickly filled and emptied. A disadvantage is that random packing has a lower packing density than when the elements are packed in a regular array. Regular arrangements can include face-centered cubic (FCC) or hexagonal close pack (HCP) arrays. While the lower packing density can have the advantage of lowering flow resistance, this benefit comes at a significant cost of increased reactant volume, commensurately lower flow rates, and non-uniformity of flow. FIGS. 1A and 1B show a simulation of the flow profile through an ALD reactor 10. FIG. 1A shows the reactor filled with a HCP array of spheres 12. FIG. 1B shows a top, cross-sectional view of the spheres in FIG. 1A. Throughout the specification, the shading indicates velocity, with lighter shading indicating low velocity and darker shading indicating high velocity. Thus, in FIGS. 1A and 1B, the lighter shading between the spheres indicates low velocity, the medium shading around the perimeter of the cluster of spheres indicates medium velocity, and the darker shading at the very outer perimeter of the reactor indicates high velocity. The flow conditions are an outlet pressure of 1 Torr, 20 sccm (standard cubic centimeters per minute) of nitrogen, and the reactor temperature is 200° C. The simulation was made with Solidworks 2012 flow simulator sold by Dassault Systemes Corp, Velizy, France.